The present invention relates to chemical mixture analysis and, more particularly, relates to a practical approach for achieving improved selectivity for capillary gas chromatography.
Gas chromatography is an analytical technique for separating compounds from a mixture based on their volatilities and polarities. Gas chromatography thus provides both qualitative and quantitative information for the individual compounds present in a sample. These compounds move through a gas chromatography column as gases, either because the compounds are normally gases or because they have been heated and vaporized into a gaseous state. The compounds partition between a stationary phase, which can be either solid or liquid, and a mobile phase, which is typically gas.
As is known in the art, the introduction of the fused silica, wallcoated, capillary gas chromatography column in 1979 represented a paradigm shift in the analysis of volatile and semi-volatile organic compounds. However, with the introduction of the fused-silica capillary column, the emphasis gradually shifted towards the development of more universal columns with greater resolving power for a wider variety of compound mixtures.
Long capillary columns (generally greater than 20 m long) generally provide greater resolving power and, thus, the complete separation of more complex mixtures with analysis times comparable to those achieved with packed columns.
Briefly, as used herein, xe2x80x9cselectivityxe2x80x9d generally refers to the pattern of peaks produced from the gas chromatography column and the values of the retention factor ratios for all component pairs. Retention factor is equal to the difference between the retention time and the column void time ratioed to the void time. The greater the selectivity of the column for a specified component pair, the more separation between the corresponding peaks of the chromatogram can be achieved.
On the other hand, shorter capillary columns (generally 5-20 m long), which are operated at relatively high carrier gas flow rates, have been used to obtain faster mixture separations. However, in exchange for faster mixture separations, resolving power is sacrificed. As should be appreciated, if these techniques for high-speed gas chromatography are to be useful for more complex mixtures, selectivity or resolving power must be improved.
Accordingly, there exists a need in the relevant art to provide a method and apparatus whereby a wide variety of compounds may be quickly separated without significantly affecting the resolving power. Furthermore, there exists a need in the relevant art to provide a method and apparatus of capillary column gas chromatography that is capable of varying the selectivity during the course of the compound analysis to provide improved selectivity capability for various specific compound pairs. Still further, there exists a need in the relevant art to provide a method and apparatus of pulsed carrier gas flow modulation for improved selectivity of gas chromatography. Additionally, there exists a need in the relevant art to provide a method of capillary gas chromatography that overcomes the disadvantages of the prior art.
According to the principles of the present invention, a gas chromatography system is provided having an advantageous construction and method of using the same. The gas chromatography system includes a computer-controlled venting valve system that delivers pressurized pulses to a column junction point of two series-coupled columns having different stationary-phase chemistries. Each pressurized pulse causes a differential change in the carrier gas velocities in the two columns, which lasts for the duration of the pressurized pulse. Whereby, the pressurized pulse selectively increases the separation of a component pair that exhibits separation at the exit of the first column, but otherwise co-elutes from the column ensemble.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiments of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.